Sometimes I have found after hosting new web project to new Linux Server, so many services stop working like webmail, ssl, WHM, cpanel.
Later, I figure it out that, most of it because Firewall blocks them as there are no authorized port described / configured in the firewall.
It always so tough to remember all that ports that are very common & standard to set them from beginning.

I am adding some standard ports that you should turn on in your firewall for your linux server.